The long-term goal of this work is the elaboration and study of proteins containing synthetic amino acids at specific, predetermined positions. These are to be incorporated by read through of nonsense codons with misacylated suppressor tRNAs. For the period of requested support, we propose to prepare and study analogues of firefly luciferase, MAP kinase ERK2, and DNA polymerase beta. Also of interest is the exploitation of a new method for introducing a broader range of amino acid analogues into the derived proteins, and the elaboration of the modified proteins in an intact cellular system